Maybe you have seen thermal expansion tanks at your local home improvement store, installed near your neighbor’s new water heater, or maybe your plumber has told you that you need one. If so, you may be wondering what a thermal expansion tank is and whether you really do need one. After all, you’ve gotten along for years without one so they must not be necessary, right?
The answer depends on whether you have a closed or open plumbing system. A thermal expansion tank should be installed in closed systems with traditional water heaters.
Until recently, most plumbing systems were open systems. In an open system, water normally flows from the municipal water system into your house, but under some circumstances, it is possible for water to flow “backwards” from your house into the municipal water system.
Responding to concerns about water system contamination, many municipalities have installed backflow prevention valves, also called check valves or one-way valves, to prevent water from flowing back into the municipal water system. These valves are generally installed on your water meter and are designed to protect the municipal water supply from accidental or intentional contamination. Systems with backflow prevention valves are called closed systems
The water in your water heater expands as it heats up. In an open system, this was not a problem because the water could flow backwards out of your house to provide for the water expansion. However, in a closed system there is nowhere for the water to go and instead the pressure in your pipes increases as the water heats up. To stop this from happening, thermal expansion tanks are installed to relieve the pressure build up.
